This invention relates to the production of magnetic disks using glass substrates.
Most magnetic disks manufactured today comprise either aluminum or glass substrates. Glass substrates for magnetic disks are typically formed by the following process:
1. A sheet of glass 1 (FIG. 1) is prepared, e.g. by the xe2x80x9cfloat methodxe2x80x9d.
2. Individual xe2x80x9csquaresxe2x80x9d of glass (e.g. square 2 of FIG. 2) are cut out of sheet 1. This is typically done using a diamond scribing process.
3. The xe2x80x9csquaresxe2x80x9d of glass (e.g. square 2) are cut into disk-shaped substrates (e.g. substrate 3 of FIG. 3). During this cutting operation, a circular region is also cut out of an inner portion of disk shaped substrate 3 to leave a circular hole 4. After the disk is completed, hole 4 facilitates mounting of the disk on a spindle within a disk drive.
4. Chamfers (such as chamfer 6 of FIG. 4) are formed at the inner diameter (xe2x80x9cIDxe2x80x9d) and outer diameter (xe2x80x9cODxe2x80x9d) of each disk 3. This is generally a difficult operation.
It is accomplished using a grinding process (which typically damages the glass material) followed by polishing chamfer 6 (to try to remove the damage).
5. After chamfer 6 is formed, the disk is subjected to multiple lapping and polishing steps. These multiple steps are necessary for at least two reasons. First, during the step of providing a chamfer, the glass is damaged, and this damage must be removed. (Merely polishing the chamfer will not remove all of the damage.) Second, during various portions of the manufacturing process, the glass is typically stacked and unstacked. During stacking an unstacking, as well as cutting, the glass is typically damaged due to glass particles that are generated during the cutting steps. These glass particles lodge on the glass prior to stacking and damage the glass.
Forming chamfers, lapping and polishing substrates constitute a great expense during substrate manufacturing. It would be highly desirable to avoid or minimize these expenses.
A method in accordance with a first embodiment of the invention comprises the acts of a) applying a protective layer of material on a glass sheet; b) cutting the glass sheet into squares; and c) stacking the glass squares for further processing. In one embodiment, the further processing includes cutting the squares into substrates. Of importance, the protective layer prevents glass particles from damaging the surface of the substrates during the further processing. Thereafter, the substrates are unstacked and the protective layers are removed.
One important feature of this embodiment of our invention is that because the glass substrate surfaces are not damaged by glass particles, it is not necessary to subject them to multiple lapping and polishing steps that would otherwise be needed to achieve defect-free surfaces. Therefore, we can use this process to provide much less expensive substrates than were previously available.
A process in accordance with a second embodiment of our invention comprises rounding the corners of the glass substrates without forming chamfers. In one embodiment, this is accomplished by applying an edge polishing step on the stacked substrates. The protective layers on the substrates are softer than glass. The ID and OD of the stacked glass substrates are subjected to a polishing process, e.g. using a slurry. The portion of the protective layers at the ID and OD are removed first. This exposes portions of the glass substrates at the corner of the ID and OD to the slurry, and permits the slurry to round the substrate corners. We have discovered it is not necessary to form chamfers in a glass substrate, and that simply rounding the substrate corners suffices in a manufacturing process. In particular, the substrate can be safely handled, although it lacks chamfers, without damaging the substrate. Also, because the edges are rounded and not sharp, the substrates can be safely handled by people and associated process equipment. Further, by not chamfering the substrate corners, we avoid the expense and damage to the glass material associated with chamfering.
After the substrates are machined as discussed above, they are used for magnetic disk, manufacturing. In one embodiment, the substrates are destacked, the protective layers are removed, and the substrates are subjected to brief, minimal polishing. Because the substrates were protected during stacking and during cutting of the glass squares into substrates, the damage which would otherwise be caused by glass particles generated during the initial cutting process is not present. Therefore, only minimal polishing is required at this point in the manufacturing process. This minimal polishing leaves the initial substrate smoothness substantially undisturbed. Further, the minimal polishing removes less substrate material, and is much less expensive, than prior art lapping and polishing steps used to manufacture prior art disk substrates. After the minimal polishing step the substrates are then cleaned.
Optionally, the substrates are then subjected to a chemical strengthening step. After chemical strengthening the substrates are again cleaned.
Thereafter, the various layers of the magnetic disk are deposited on the substrate, e.g. by sputtering. For example, an underlayer such as Cr, a Cr alloy, NiP, or NiAl is sputtered onto the substrates, followed by a magnetic Co alloy and a protective overcoat.
The glass used to practice this invention can be any type of glass, e.g. a borosilicate glass, or an alumina silicate glass. In lieu of glass, the invention can be practiced using glass ceramic substrates.